Prostacyclin (PGE1), an arachidonic acid metabolite, has been evaluated for its efficacy in the treatment of pulmonary hypertension and for its use in reducing intra-pulmonary shunting in ARDS. While the intravenous application PGE1, or alprostadil, can cause a decrease not only of the pulmonary but also of the systemic vascular tone, the aerosolized PGE1 results in a selective pulmonary vasodilation without affecting the systemic blood pressure. Furthermore, aerosolized PGE1 can improve gas exchange and pulmonary shunt in clinical settings of impaired ventilation/perfusion ratio as it occurs in ARDS, due to the redistribution of pulmonary blood flow from non-ventilated to ventilated, aerosol accessible lung regions.